I have been happy with mine. I was considering this lens vs the 24-120 and selected the Sigma because a local dealer had it in stock and I was able to try it on my D700 and D4. I am pleased with the Sigma 35 f/1.4, and therefore felt comfortable purchasing this Sigma without waiting for the 24-120. The OP question related to the D800 which I do not have.

I do not think you can clearly say one is better. Most reviews suggest Sigma is sharper and probably more solidly built at the expense of weight, has worse vignetting, slightly less distortion, autofocus which may be easier to fool, lacks weather sealing, and requires an 82mm filter instead of the 77 which fits most of the rest of the stuff in the bag, has zoom and focus rings switched in position from familiar 70-200 and 24-70, zoom ring rotation is opposite direction of Nikon (although I have not used the 24-120). Sigma focus and firmware can be adjusted with a USB dock which I do not own and have not needed.

Conclusions of those with much greater stature than mine are generally that it is a worthwhile consideration, but not so great an improvement that one consider switch from the Canon L or the Nikon. Prices of the Canon and Nikon (for 2 more days) are now so close that that is not a major factor for first time purchase.

Scored better than Nikon 24-120 on DXO, but was compared to the Sigma on a Canon body with lower res sensor and of likely dubious value. Lenstip appears to favor it, but not so much as to recommend a switch, same with Mansurov.